Adrenomedullin is a novel peptide that exerts a potent, dose-dependent and long-lasting hypotensive effect. In human plasma, adrenomedullin consists of two molecular forms: mature and immature. Immature adrenomedullin is much less bioactive than mature adrenomedullin. Although a gradual increase in plasma adrenomedullin has been reportedly observed as pregnancy progressed, mature adrenomedullin has not been examined. The aim of this study was to elucidate the plasma level of mature adrenomedullin in pregnant women. We measured the concentrations of mature adrenomedullin in ten pregnant women in the first trimester, ten pregnant women in the third trimester, and ten non-pregnant controls with the immunoradiometric assay. The mean concentration of mature adrenomedullin was significantly increased in pregnant women in the first trimester compared to age-matched non-pregnant subjects (p < 0.05). The mean concentration of mature adrenomedullin was significantly increased in pregnant women in the third trimester compared with pregnant women in the first trimester (p < 0.005). Our study demonstrated that concentrations of mature adrenomedullin were elevated in pregnant women compared with non-pregnant women and its concentration in the third trimester was significantly higher than that in the first trimester.